Pryometer for laser annealing system compatible with amorphous carbon optical absorber layer

ABSTRACT

In a laser annealing system for workpieces such as semiconductor wafers, a pyrometer wavelength response band is established within a narrow window lying between the laser emission band and a fluorescence emission band from the optical components of the laser system, the pyrometer response band lying in a wavelength region at which the optical absorber layer on the workpiece has an optical absorption coefficient as great as or greater than the underlying workpiece. A multi-layer razor-edge interference filter having a 5-8 nm wavelength cut-off edge transition provides the cut-off of the laser emission at the bottom end of the pyrometer response band.

BACKGROUND

Thermal processing is required in the fabrication of silicon and othersemiconductor integrated circuits formed in silicon wafers or othersubstrates such as glass panels for displays. The required temperaturesmay range from relatively low temperatures of less than 250° C. togreater than 1000°, 1200°, or even 1400° C. and may be used for avariety of processes such as dopant implant annealing, crystallization,oxidation, nitridation, silicidation, and chemical vapor deposition aswell as others.

For the very shallow device features required for advanced integratedcircuits, it is desired to reduce the total thermal budget in achievingthe required thermal processing. The thermal budget may be considered asthe total time at high temperatures necessary to achieve the desiredprocessing results (e.g., dopant activation level). The time that thewafer needs to stay at the highest temperature can be very short.

Rapid thermal processing (RTP) uses radiant lamps which can be veryquickly turned on and off to heat only the wafer and not the rest of thechamber. Pulsed laser annealing using very short (about 20 ns) laserpulses is effective at heating only the surface layer and not theunderlying wafer, thus allowing very short ramp up and ramp down rates.

A more recently developed approach in various forms, sometimes calledthermal flux laser annealing or dynamic surface annealing (DSA), isdescribed by Jennings et al. in U.S. Pat. No. 6,987,240 and incorporatedherein by reference in its entirety. Markle describes a different formin U.S. Pat. No. 6,531,681 and Talwar yet a further version in U.S. Pat.No. 6,747,245.

The Jennings and Markle versions use CW diode lasers to produce veryintense beams of light that strikes the wafer as a thin long line ofradiation. The line is then scanned over the surface of the wafer in adirection perpendicular to the long dimension of the line beam.

SUMMARY

A thermal processing system includes a source of laser radiationemitting at a laser wavelength, a beam splitting reflective memberarranged to receive the laser radiation, and beam projection opticsdisposed between one side of the reflective member and a substratesupport capable of holding a substrate to be processed. A projectionoptical path for the laser radiation extends from the reflecting member,through the projection optics and toward the substrate support. Thesystem further includes a pyrometer on an opposite side of thereflective member and responsive to a pyrometer wavelength range orresponse band, and a pyrometer optical path extending through thereflective member and to the pyrometer. An amorphous carbon opticalabsorber layer covers the surface of the substrate being processed. Thesystem further includes a pyrometer passband filter in the pyrometeroptical path having a narrow passband lying in a wavelength windowbetween the laser emission band and an emission band of fluorescence ofthe optical components of the projection and pyrometer optical paths.The pyrometer passband filter blocks the fluorescence emission from thepyrometer optical path. The narrow pyrometer filter passband window liesin a wavelength range within which the amorphous carbon layer has asubstantial extinction coefficient on the order of or exceeding that ofthe underlying integrated circuit features. A multiple thin film razoredge filter in the pyrometer optical path blocks the laser emission bandfrom the pyrometer optical path.

In one embodiment, the source of laser radiation includes an array oflaser emitters. In one embodiment, the beam projection optics projects aline beam of radiation of the laser wavelength onto a substrate planeover the substrate support, and the system further includes a line beamscanning apparatus having a fast axis transverse to the line beam.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited embodiments of theinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings. It is to be noted, however, that the appendeddrawings illustrate only typical embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

FIG. 1 is an orthographic representation of a thermal flux laserannealing apparatus employed in the present invention.

FIGS. 2 and 3 are orthographic views from different perspectives ofoptical components of the apparatus of FIG. 1.

FIG. 4 is an end plan view of a portion of a semiconductor laser arrayin the apparatus of FIG. 1.

FIG. 5 is a schematic diagram of a system including the features ofFIGS. 2-4.

FIG. 6 is a graph depicting the spectrum of radiation presented to thepyrometer optical path, including a laser radiation peak at 810 nm and afluorescence peak at 950 nm.

FIG. 7 is a graph depicting the extinction coefficient of the amorphouscarbon optical absorber layer on the wafer as a function of wavelength.

FIG. 8 is a graph depicting the response of a razor-edge long wavelengthpass filter used in the system of FIG. 5.

FIG. 9 is a graph depicting the response of the pyrometer bandpassfilter in the system of FIG. 5.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The drawings in the figures are all schematic and not toscale.

DETAILED DESCRIPTION

One embodiment of the apparatus described in the above-referenced U.S.patent by Jennings et al. is illustrated in the schematic orthographicrepresentation of FIG. 1. A gantry structure 10 for two-dimensionalscanning includes a pair of fixed parallel rails 12, 14. Two parallelgantry beams 16, 18 are fixed together a set distance apart andsupported on the fixed rails 12, 14 and are controlled by anunillustrated motor and drive mechanism to slide on rollers or ballbearings together along the fixed rails 12, 14. A beam source 20 isslidably supported on the gantry beams 16, 18, and may be suspendedbelow the beams 16, 18 which are controlled by unillustrated motors anddrive mechanisms to slide along them. A substrate 22 to be processed,which may be a silicon wafer 22 for example, is stationarily supportedbelow the gantry structure 10. The beam source 20 includes a laser lightsource and optics to produce a downwardly directed fan-shaped beam 24that strikes the wafer 22 as a line beam 26 extending generally parallelto the fixed rails 12, 14, in what is conveniently called the slowdirection. Although not illustrated here, the gantry structure furtherincludes a Z-axis stage for moving the laser light source and optics ina direction generally parallel to the fan-shaped beam 24 to therebycontrollably vary the distance between the beam source 20 and the wafer22 and thus control the focusing of the line beam 26 on the wafer 22.Exemplary dimensions of the line beam 26 include a length of 1 cm and awidth of 66 microns with an exemplary power density of 220 kW/cm².Alternatively, the beam source and associated optics may be stationarywhile the wafer is supported on a stage which scans it in twodimensions.

In typical operation, the gantry beams 16, 18 are set at a particularposition along the fixed rails 12, 14 and the beam source 20 is moved ata uniform speed along the gantry beams 16, 18 to scan the line beam 26perpendicularly to its long dimension in a direction conveniently calledthe fast direction. The line beam 26 is thereby scanned from one side ofthe wafer 22 to the other to irradiate a 1 cm swath of the wafer 22. Theline beam 26 is narrow enough and the scanning speed in the fastdirection fast enough that a particular area of the wafer is onlymomentarily exposed to the optical radiation of the line beam 26 but theintensity at the peak of the line beam is enough to heat the surfaceregion to very high temperatures. However, the deeper portions of thewafer 22 are not significantly heated and further act as a heat sink toquickly cool the surface region. Once the fast scan has been completed,the gantry beams 16, 18 are moved along the fixed rails 12, 14 to a newposition such that the line beam 26 is moved along its long dimensionextending along the slow axis. The fast scanning is then performed toirradiate a neighboring swath of the wafer 22. The alternating fast andslow scanning are repeated, perhaps in a serpentine path of the beamsource 20, until the entire wafer 22 has been thermally processed.

The optics beam source 20 includes an array of lasers. An example isorthographically illustrated in FIGS. 2, 3 and 5, in which laserradiation at about 810 nm is produced in an optical system 30 from twolaser bar stacks 32, one of which is illustrated in end plan view inFIG. 4. Each laser bar stack 32 includes a number of (e.g., fourteen)parallel bars 34, generally corresponding to a vertical p-n junction ina GaAs semiconductor structure, extending laterally about 1 cm andseparated by about 0.9 mm. Typically, water cooling layers are disposedbetween the bars 34. In each bar 34 are formed a number of (e.g.,forty-nine) emitters 36, each constituting a separate GaAs laseremitting respective beams, each beam having different divergence anglesin orthogonal directions. The illustrated bars 34 are positioned withtheir long dimension extending over multiple emitters 36 and alignedalong the slow axis and their short dimension corresponding to the lessthan 1-micron p-n depletion layer aligned along the fast axis. The smallsource size along the fast axis allows effective collimation along thefast axis. The divergence angle is large along the fast axis andrelatively small along the slow axis.

Returning to FIGS. 2, 3 and 5, two arrays of cylindrical lenslets 40 arepositioned along the laser bars 34 to collimate the laser light in anarrow beam along the fast axis. They may be bonded with adhesive on thelaser stacks 32 and aligned with the bars 34 to extend over the emittingareas 36.

The optics beam source 20 can further include conventional opticalelements. Such conventional optical elements can include an interleaverand a polarization multiplexer, although the selection by the skilledworker of such elements is not limited to such an example. In theexample of FIGS. 2, 3 and 5, the two sets of beams from the two barstacks 32 are input to an interleaver 42, which has a multiple beamsplitter type of structure with specified coatings on two internaldiagonal faces, e.g., reflective parallel bands, to selectively reflectand transmit light. Such interleavers are commercially available fromResearch Electro Optics (REO). In the interleaver 42, patterned metallicreflector bands are formed in angled surfaces for each set of beams fromthe two bar stacks 32 such that beams from bars 34 on one side of thestack 32 are alternatively reflected or transmitted and therebyinterleaved with beams from bars 34 on the other side of the stack 32which undergo corresponding selective transmission/reflection, therebyfilling in the otherwise spaced radiation profile from the separatedemitters 36.

A first set of interleaved beams is passed through a quarter-wave plate48 to rotate its polarization relative to that of the second set ofinterleaved beams. Both sets of interleaved beams are input to apolarization multiplexer (PMUX) 52 having a structure of a doublepolarization beam splitter. Such a PMUX is commercially available fromResearch Electro Optics. First and second diagonal interface layers 54,56 cause the two sets of interleaved beams to be reflected along acommon axis from their front faces. The first interface 54 is typicallyimplemented as a dielectric interference filter designed as a hardreflector (HR) while the second interface 56 is implemented as adielectric interference filter designed as a polarization beam splitter(PBS) at the laser wavelength. As a result, the first set of interleavedbeams reflected from the first interface layer 54 strikes the back ofthe second interface layer 56. Because of the polarization rotationintroduced by the quarter-wave plate 48, the first set of interleavedbeams passes through the second interface layer 56. The intensity of asource beam 58 output by the PMUX 52 is doubled from that of either ofthe two sets of interleaved beams.

Although shown separated in the drawings, the interleaver 42, thequarter-wave plate 48 and the PMUX 52 and its interfaces 54, 56, as wellas additional filters that may be attached to input and output faces aretypically joined together by a plastic encapsulant, such as a UV curableepoxy, to provide a rigid optical system. An important interface is theplastic bonding of the lenslets 40 to the laser stacks 32, on which theymust be aligned to the bars 34. The source beam 58 is passed through aset of cylindrical lenses 62, 64, 66 to focus the source beam 58 alongthe slow axis.

A one-dimensional light pipe 70 homogenizes the source beam along theslow axis. The source beam, focused by the cylindrical lenses 62, 64,66, enters the light pipe 70 with a finite convergence angle along theslow axis but substantially collimated along the fast axis. It has ashort dimension along the slow axis and a longer dimension along thefast axis.

The source beam output by the light pipe 70 is generally uniform.Anamorphic optics 80, 82 depicted in FIG. 5 focus the source beam intothe line beam of desired dimensions on the surface of the wafer 22.

In one embodiment, the same optics that focus the laser source light onthe wafer also direct thermal radiation emitted from the neighborhood ofthe line beam 26 on the wafer 22 in the reverse direction to a pyrometer60, schematically illustrated in FIG. 5. As will be described below, thepyrometer is responsive to a limited wavelength range, which is referredto in this specification as the pyrometer response band, the centerwavelength in this band being referred to in this specification as thepyrometer wavelength. The two PMUX interfaces 54, 56 are designed topass the pyrometer wavelength irrespective of its polarization. Theoptics are generally reciprocal and thus in the reverse direction detectonly a small area of the wafer 22 on or very near to the line beam 26and optically expands that image to an area generally having a size ofthe emission face of the bar stacks. Although with small laser-beamannealing, there is significant variation of surface layer temperatureseven in the immediate neighborhood of the line beam 26, the nature ofthe blackbody radiation spectrum causes the hottest areas to dominatethe thermally emitted radiation.

The variation in temperature across the area of the wafer illuminated bythe powerful laser line beam arises from the presence of differentfeatures on the wafer surface that absorb the laser radiation atdifferent rates (because they have different extinction coefficients) orhave uneven surfaces that reflect in different directions. In order toobtain a more uniform heating of the wafer during the DSA laserannealing, the entire wafer surface is covered with an opticalabsorption layer prior to laser annealing. The optical absorption layerin one embodiment is an amorphous carbon layer because it has asubstantial absorption coefficients at the laser wavelength (810 nm) andat the pyrometer wavelength (e.g., 950 nm) exceeding those of theunderlying integrated circuit features on the wafer. Therefore, theuniform heat absorption of the amorphous carbon layer predominates overthe non-uniformities of the underlying integrated circuit structures.Moreover, the blackbody radiation uniformly emitted by the amorphouscarbon layer at the pyrometer wavelength predominates over radiationemitted by the non-uniform integrated circuit elements underlying theamorphous carbon layer. This prevents underlying integrated circuitpattern effects from distorting the pyrometer measurements of wafertemperature.

The pyrometer 60 includes an optical detector 61, such as a photodiode,and an optical pyrometer bandpass filter 63. The pyrometer filter 63helps establish the pyrometer response band. Conventionally, onepossible pyrometer response band could be centered at 1550 nm. However,the amorphous carbon optical absorber layer covering the wafer foruniform absorption, as well as the Si substrate itself, does not absorbwell (has a lower extinction coefficient) at such a long wavelength, andtherefore does not improve uniformity of absorption and blackbodyradiation emission. The surface emissivity at this wavelength alsochanges with the wafer temperature. Therefore, 1550 nm is not a goodchoice for the pyrometer response band.

Another possible choice is to center the pyrometer response band at theshorter wavelength 950 nm with a bandwidth of a few tens of nm. This maybe achieved by providing the pyrometer passband filter 63 with apassband center wavelength near 950 nm. At this shorter wavelength, theamorphous carbon optical absorber layer absorbs well and thereforeprovides uniform absorption of the laser radiation across the wafersurface.

The output of the photodetector 61 is supplied to a source controller65, which converts the detected photocurrent to a wafer temperature andcompares it to a desired temperature and thereby adjusts the powersupplied to the laser bars 32 to increase or decrease their opticaloutput in the direction of the desired wafer temperature.

The GaAs or other semiconductor lasers have a fairly wide spectrum oflow-level spontaneous emission that typically overlaps the pyrometerwavelength response band. As a result of the spontaneous emission, whichthe pyrometer filter 63 does not block at the pyrometer wavelength, thephotodetector 61 would detect both: (a) the wafer blackbody radiation atthe pyrometer wavelength and (b) the portion of the laser sourcespontaneous emission at the pyrometer wavelength, in the absence ofadditional filtering.

The pyrometer performance can be improved by filtering out the lasersource spontaneous radiation at the pyrometer wavelength with a notchfilter 67 placed between the bar stacks 32 and the interleaver 42, orwith a notch filter 68 placed between the interleaver 42 and the PMUX52. The notch filter 67 or the notch filter 68 blocks the sourceradiation at the pyrometer wavelength, e.g. 950 nm, and passes at leastthe laser radiation at 810 nm. The ratio of the transmission coefficientof the laser wavelength to that of pyrometer wavelength should beseveral orders of magnitude. A minimum requirement of the filters 67, 68is that they block wavelengths longer than the laser wavelength (e.g.,longer than the laser wavelength 810 nm), although radiation at shorterwavelengths does not inherently degrade the pyrometer. The notch filters67, 68 may be implemented as interference filters coated on either theinterleaver 42 or the PMUX 52, although they may be implemented as standalone filters.

Filtering Out Spurious Noise at the Pyrometer

The pyrometer 60 experiences a high level spurious background signalwhich distorts the temperature measurement function of the pyrometer.This poses a severe problem in the closed feedback control loop of thesource controller 65, since the spurious background signal varies withlaser power and wafer surface reflectivity non-uniformity. We havediscovered that this background signal is caused by the fluorescence ofthe optical components such as the beam splitter 52, the lenses 62, 64,66 and other components illustrated in FIG. 5. The fluorescencebackground signal is particularly acute when the optical components areformed of a fused quartz material. One example of such material is anoptical glass material sold under the registered trademark Infrasil®owned by Heraeus Quarzglas G.M.B.H. This material is sold by HeraeusQuartz America, L.L.C. Other related materials which may be moreexpensive than the Infrasil® material may have somewhat lowerfluorescence but nevertheless to emit fluorescence that hamperstemperature measurement. The fluorescence of the Infrasil® material hasa peak amplitude near the conventional choice of pyrometer wavelength,950 nm, which can equal or exceed the amplitude of the blackbodyradiation from the wafer at the pyrometer wavelength of 950 nm. Sincethe Infrasil® fluorescence has a maximum or peak at the pyrometerwavelength (950 nm), the fluorescence is passed by the pyrometer optics,to become a strong spurious background signal that distorts temperaturemeasurements by the pyrometer.

The spectrum of radiation present in the system is depicted in the graphof FIG. 6. The peak near 810 nm corresponds to the CW laser radiation,this emission being within a band lying between about 805 nm and 815 nm,according to the graph of FIG. 6. The peak near 950 nm corresponds tothe fluorescence of the optical components. This fluorescence has a dipnear 1100 nm. Therefore, one possible approach is to move the pyrometerresponse band (the passband of the filter 63) to about 1100 nm, to avoidmuch of the fluorescence of the optical components. Such an approach,however, conflicts with the purpose of the amorphous carbon opticalabsorber layer covering the wafer. This is because the amorphous carbonlayer has low optical absorption at this longer wavelength (andtherefore correspondingly low blackbody emission at this wavelength),and therefore provides less improvement in uniformity of the temperaturemeasurement. The situation is depicted in the graph of FIG. 7, showingthe extinction coefficient of the amorphous carbon layer as a functionof wavelength. The extinction coefficient is an indicator of theefficiency with which the amorphous carbon material absorbs radiation.FIG. 7 shows that the amorphous carbon layer extinction coefficientfalls to a very low level above a wavelength of 1000 nm. Therefore,increasing the pyrometer wavelength above 950 is not a practicalapproach.

We have discovered that there is a 40 nm wide window between the 805nm-815 nm laser emission band and the onset of the fluorescence (fromthe optical components) at about 855 nm or 860 nm. The location of this40 nm window is indicated in FIG. 6. Within this 40 nm window, there islittle or no laser emission and little or no fluorescence from theoptical components. Moreover, referring to FIG. 7, within this window(i.e., between 815 nm and 855 nm), the amorphous carbon optical absorberlayer has a reasonably strong extinction coefficient, e.g., betweenabout 0.05 and 0.10. Therefore, restricting the pyrometer response bandto fit within the narrow 40 nm window (FIG. 6), places it in a region ofhigh extinction coefficient of the amorphous carbon layer, lowfluorescence emission from the optical components and insignificantlaser emission. By restricting the pyrometer response band to a regionof relatively high extinction coefficient of the amorphous carbon layer,the amorphous carbon layer on the wafer absorbs more of laser radiation,and therefore its uniform absorption dominates, providing a uniformannealing process across the wafer surface. In the pyrometer responseband of the 40 nm-wide window, the optical absorber (amorphous carbon)layer has an optical absorption coefficient that is at least at great asor greater than the optical absorption coefficient of the underlyingsubstrate.

However, the 40 nm window is too close to the laser emission band(centered around the laser wavelength) for a typical filter used toimplement the pyrometer response band filter 63. In particular, thewavelength difference between the laser radiation peak at 810 nm and thebeginning of the 40 nm window at 815 nm is extremely small. It istherefore difficult to block the laser radiation at 810 nm without alsoblocking the desired blackbody radiation (pyrometer signal) at 840 nm.This problem is solved by providing a razor-edge optical filter 72 inthe system of FIGS. 2 and 5, so that the conventional passband filter 63is not required to block the laser radiation at 810 nm. In fact, theresponse of the conventional passband filter 63 may be such as to admitat least much of the laser radiation and thereby avoid inadvertentlyblocking or partially blocking the pyrometer wavelengths in thepyrometer response band. Blocking of the laser radiation is performedinstead by the razor edge filter 72. The razor-edge optical filter 72 isa long wavelength pass filter, and has the optical response depicted inthe graph of FIG. 8. The cut-off wavelength of the razor edge filter 72is at about 815 nm. The razor edge filter 72 has an edge transition froma transmittance of nearly 100% (full transmission or transparency) abovethe cut-off wavelength to only 10⁻⁶ at (and below) the cut-offwavelength of 815 nm. This edge transition is extremely narrow, onlyfive to eight nanometers wide. As a result, the razor edge filter blocksthe laser radiation at 810 nm but is fully transparent to the pyrometerwavelength above 815 nm. Such an extremely sharp response is obtained byconstructing the razor edge filter 72 as an interference filter.

Interference filters are multilayer thin-film devices. They can bedesigned to function as an edge filter or bandpass filter. In eithercase, wavelength selection is based on the property of destructive lightinterference. In such a filter, incident light is passed through manypairs of coated reflecting surfaces. The distance between the reflectivecoatings determines which wavelengths destructively interfere and whichwavelengths are in phase and will ultimately pass through the coatings.The gap between the reflecting surfaces is a thin film of dielectricmaterial called a spacer. It has a thickness of one-half wave at thedesired peak transmission wavelength. The reflecting layers can consistof several film layers, each of which is a quarter wavelength thick.This sandwich of quarterwave layers is made up of an alternating patternof high and low index material, usually zinc sulfide and cryolite,respectively. Together, the quarterwave coatings forming the reflectivelayer is called a stack. There may be many stacks in the filter 72 toachieve the sharp cut-off response depicted in FIG. 8. Such a razor-edgeoptical filter is available from Semrock, of Rochester, N.Y.

Referring again to FIG. 5, the conventional bandpass filter 63 isselected to have a passband centered at about 840 nm, i.e., at about thecenter of the 40 nm window depicted in FIG. 6. In one implementation,the 840 nm passband filter 63 has a full width half maximum of 10 nm,roughly corresponding to the response depicted in the graph of FIG. 9.Being centered at 840 nm, it effectively blocks the fluorescencebackground emission from the optical components corresponding to the 950nm peak in the graph of FIG. 6. Thus, the pyrometer response band isestablished within the 40 nm window of FIG. 6 by the razor-edge filter72 blocking radiation below about 815 nm and the pyrometer passbandfilter 63 blocking radiation above about 860 nm. In an alternativeembodiment, the system of FIG. 5 further includes an optional notchfilter 74 that blocks the laser emission at 810 nm.

While the foregoing is directed to embodiments of the invention, otherand further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method of laser-annealing a substrate, comprising: emitting laserradiation at a laser wavelength between about 805 nm and about 815 nm;passing said laser radiation in a forward direction through a beamsplitter and projection optics to a substrate, at least one of the beamsplitter and the projection optics comprising material having afluorescence wavelength band lying at or above about 855 nm and abovesaid laser wavelength; imaging the region of the substrate illuminatedby the laser radiation onto a pyrometer through a pyrometer opticalpath, and admitting radiation through said pyrometer optical path lyingin a pyrometer response band above the laser wavelength and below thefluorescence wavelength band; and blocking radiation wavelengths in thepyrometer optical path below a cut-off wavelength edge transition andpassing radiation wavelengths above said wavelength edge transition,said wavelength edge transition being less than 10 nm wide, saidwavelength edge transition being above said laser wavelength and belowsaid pyrometer response band; blocking radiation wavelengths in thepyrometer optical path that lie within and above the fluorescencewavelength band.
 2. The method of claim 1 further comprising coating thesubstrate with an optical absorber layer comprising an amorphous carbonmaterial prior to exposing the substrate to the laser radiation, saidpyrometer response band lying in a wavelength range at which saidamorphous carbon layer has an absorption coefficient at least as greatas said substrate.
 3. The system of claim 2 wherein said pyrometerresponse band lies in a wavelength range at which said amorphous carbonlayer has an absorption coefficient greater than that of said substrate.4. The system of claim 2 wherein said edge transition is between about 5nm and about 8 nm wide, wherein said blocking the wavelengths below saidedge transition comprises attenuating the wavelengths below said edgetransition by a factor on the order of 10⁻⁶.
 5. The system of claim 4wherein said laser wavelength and said cut-off wavelength edgetransition are separated by about 5 nm.
 6. The system of claim 4 whereinsaid laser wavelength is about 810 nm, said fluorescence wavelength bandis in a wavelength range from about 860 nm and peaking at about 950 nm,said pyrometer response band lies between about 820 nm and 860 nm, andsaid cut-off wavelength edge transition is near 815 nm.
 7. A method oflaser-annealing a substrate, comprising: covering the substrate with anoptical absorber layer; emitting laser radiation at a laser wavelengthbetween about 805 nm and about 815 nm as a line beam using opticalcomponents comprising material having a fluorescence wavelength bandlying at or above about 855 nm and above said laser wavelength; andimaging the region of the substrate illuminated by the laser radiationonto a pyrometer through a pyrometer optical path, and admittingradiation through said pyrometer optical path lying in a pyrometerresponse band above the laser wavelength and below the fluorescencewavelength band while blocking radiation in the pyrometer optical pathoutside of the pyrometer response band, said pyrometer response bandlying in a wavelength region within which said optical absorber layerhas an optical absorption coefficient at least as great as that of thesubstrate.
 8. The method of claim 7 wherein the blocking radiationoutside of the pyrometer response band comprises: blocking radiationwavelengths in the pyrometer optical path below a cut-off wavelengthedge transition and passing radiation wavelengths above said edgetransition, said edge transition being less than 10 nm wide, said edgetransition being above said laser wavelength and below said pyrometerresponse band; and blocking radiation wavelengths in the pyrometeroptical path that lie within the fluorescence wavelength band.
 9. Thesystem of claim 8 wherein said edge transition is between about 5 nm andabout 8 nm wide, wherein said blocking the wavelengths below said edgetransition comprises attenuating the wavelengths below said edgetransition by a factor on the order of 10⁻⁶.
 10. The system of claim 7wherein the optical absorber layer comprises amorphous carbon.